1. Field of the Invention
This invention relates broadly in the field of electronic musical tone generators and in particular is concerned with provision for causing a loudness signal to control the spectral content of the generated tones.
2. Description of the Prior Art
An elusive goal in the design of electronic musical instruments is the ability to realistically imitate the sounds of conventional acoustic type orchestral musical instruments. The best results have been obtained for electronic musical instruments which imitate wind-blown pipe organs and harpsichords. The principal reason for obtaining good imitative results for these instruments is that they are essentially mechanical tone generators. The tone generation is automatic and the musician only actuates on-off switches. With the notable exception of these two instruments, the tone character of almost all other musical instruments is a function of certain skills possessed by the musician.
It has long been recognized that with the notable exception of conventional organ tones, almost all tones from musical instruments exhibit tone spectra which are time variant. The recognition of the characteristic time variant spectra has motivated the development of electronic musical generating systems such as those known by the generic names of "sliding formant" and "FM-synthesizer." Sliding formanttone generators constitute a class of generators which are also called subtractive synthesis. In subtractive synthesis, the fundamental tone source generates more than the desired tone spectral components and the undesired spectral components are attenuated, or filtered out, by means of some variety of frequency filter. The FM-synthesizer is of the additive variety in that FM (frequency modulation) is used to add components to a source signal which frequently consists of a simple single frequency sinusoid time function.
The imitation of acoustic orchestral musical instruments using synthesis techniques such as the sliding formant or FM-synthesizer has been of the trial and error variety. One adjusts a multiplicity of tone controls and ADSR envelope controls until an output tone is produced which is judged to be "near to" or imitative of some particular musical instrument. These techniques are definitely not of the more academic procedure in which one first analyzes the tones from a selected musical instrument, then constructs an analytical model of the tone generator, and finally uses the experimentally obtained parameters in the analytical model to synthesize tones that closely imitate the original.
The academic approach of analysis, modeling, and synthesis for musical instrument sounds is obviously not an easily implemented technique judging from the current lack of success except in a few relatively isolated instances. Part of the problem rests in the inability to adequately model many of the subtleties in tone structure imparted by the musician in an effective performance on his instrument. The musician commonly uses a technique such that the tonal structure for a given musical note varies with the loudness of the tone. Repeated notes are played with different loudness and tonal structure and this subtle difference removes the mechanical-like repeated tones produced by most electronic musical tone generators. Even very expert players are unable to repeat a given tone with precisely an identical tone spectra. In general, as the tone level becomes louder, the tone spectra increases in the number and strength of the higher harmonics. Very soft tones tend to approach tones having only a few harmonics.
Many of the problems encountered in the synthesis of realistic imitations of orchestral instruments are similar to those encountered in the implementation of speech synthesizers which attempt to imitate human speech. In a common variety of a speech synthesizer a pulse-like repetitive waveshape is used as one of the basic sound sources. This source is modified by linear and nonlinear system transformations in response to time variant system control parameters such that the net result is imitative of human speech.
An application of nonlinear system transformations in the generation of musical sounds is contained in the technical article:
Beauchamp, J., "Brass Tone Synthesis by Spectrum Evolution Matching with Nonlinear Functions." Computer Music Journal, Vol. 3, No. 2, pp 35-42 (1979).